A common method for dispensing sample liquid onto a test element for analysis is by aspiration and then by pressurized dispensing. That is, an aspirator probe is immersed into a sample liquid held in a suitable container. A partial vacuum is produced in the probe in an amount sufficient to draw up into the probe through its nozzle the required amount of liquid, and the probe is withdrawn to a station holding the test element. At that station, pressure is applied to the interior of the probe in an amount sufficient to dispense the desired amount of sample liquid out of the nozzle. To prevent cross-contamination between samples, the probe conventionally is provided with a removable and disposable "nozzle-container" which is the sole portion of the probe to contact the sample liquid.
Examples of such probes and nozzle-containers are described, e.g., in U.S. Pat. No. 3,832,135.
Conventional aspirator nozzles suffer the disadvantage that sample liquid tends to remain on the exterior surface of the nozzle when the probe is withdrawn after aspiration. Such remaining liquid interferes with subsequent dispensing if it collects in the immediate vicinity of the dispensing orifice of the nozzle. The reason is that the probe is designed to accurately dispense a predicted volume of sample liquid. Any liquid on the exterior surface of the nozzle at the orifice might also be dispensed. Alternatively, the presence of the liquid at the exterior surface might cause the dispensed quantity of liquid to perfuse up the exterior surface, rather than to move into or onto a test element. In either case, the volume of sample liquid received by the test element is altered in an unpredictable fashion.
One solution might seem to be to use a material for the exterior surface of the nozzle that is not wettable by the liquid to be dispensed. In the case of blood serum dispensing, such a material is not known to exist, due to the low contact angle of serum on the wetted surface.
Prior to this invention the problem has been dealt with by wiping the nozzle after aspiration. However, wiping means become, at worst, a potential source of contamination, and at best involve additional automated mechanisms that lower the throughput rate and increase the expense of the analyzer.
What has been desired then is an aspirator dispensing device, that eliminates the wiping heretofore needed while at the same time insures accurate volumes of dispensed sample liquid.